Rotary machines, such as motors, are operative to output, based on controlled AC (Alternating Current) power generated from DC (Direct Current) power supplied from a power source, torque, power, or the like. Control system are used to generate controlled AC power based on input DC power supplied from the power source.
Specifically, these control systems include an inverter and a controller. The controller controls switching elements of the inverter to convert a DC voltage supplied from a DC power source to a controlled AC voltage, thus supplying the controlled AC voltage to an AC motor as an example of rotary machines.
The controller of a normal type of these control systems operates in a sinusoidal control mode that converts a DC voltage input to the inverter to a sinusoidal AC voltage according to a switching frequency sufficiently higher than a drive frequency of the AC motor. This supplies a pseudo sinusoidal AC voltage to the AC motor without including harmonic voltage components.
Japanese Patent Application Publication No. 2001-78495 discloses another type of these control systems.
Specifically, the control system disclosed in the Patent Publication operates in a rectangular-control mode, i.e. single-pulse control mode, which controls the switching elements of the inverter using a relatively lower switching frequency as compared with that used in the sinusoidal control mode to convert the DC voltage to a rectangular AC voltage to be supplied to the AC motor. This increases a voltage utilization factor of the control system as compared with a control system for converting the DC voltage to a sinusoidal AC voltage. Note that the voltage utilization factor means the ratio of the magnitude of the inverter output voltage to that of the inverter input DC voltage.
In addition, another control system operates in an over-modulation control mode. The over-modulation control mode controls the switching elements of the inverter using a relatively lower switching frequency as compared with that used in the sinusoidal control mode. This converts the DC voltage input to the inverter to an AC voltage such that the amplitude of a fundamental component of the AC voltage is higher than the DC voltage within an over-modulation range of the inverter. This also increases the voltage utilization factor of the control system as compared with a control system for converting the DC voltage to a sinusoidal AC voltage.